Here is a prime example of the use of the fermenter that you want. First in answer to your question. Yes add the entire volume, unless you feel that there is a large amount of sediment from any previous pitches (assuming this was collected from a slurry). Adding the a measured volume is the only indication you have to judge your pitch rate.

Now if you had your conical you could be more selective about which yeast you collect for repitching. I know am a @$%^&*+! I hope you get your Christmas present. If you need a selling point figure out how much you can save on yeast if you brew often enough.

Hey, Darted. Maybe you should consider yeast washing? It'll help you shed dead yeast, trub, and other matter from the yeast. I've been reading up on it because I have a primary that's coming to a close and I want to retain the yeast, but it'll probably be at least a month before I brew again. As I understand it, washed yeast can sit under sterile water (or beer--good use for Bud, eh?) for a month before there's a significant loss in viability. But of course before you use them, you should wake them up with a starter. Here's the link:
http://www.wyeastlab.com/hbrew/hbyewash.htm